Morris Engelson Handout
- In US, commercial wireless systems use three FCC unlicensed bands (902-928 MHz, 2.40-2.485 GHz, and 5.725-5.850 GHz). Thes are known as the industrial, scientific, and medical (ISM) bands. The transmission is spread spectrum and coded at 1 Watt maximum power.
- Direct-sequence spread-spectrum (DSSS) replaces each data bit with multiple sub-bits (chips) that occupy the same time interval. The 'processing gain' equals the number of chips that replace each original bit. This is also the 'spreading ratio' which is the ratio of chipping rate to unspread information bandwidth. For example, GPS infomration bandwidth is 50Hz. A slow channel chipping rate of 1.023 MHz yields a 20460 spreading ratio. The fast channel is 10 times faster and wider in bandwidth.
- In the US and Japan the minimum permitted processing gain is 10. Each original data bit must be replaced by at least 10 sub-bit chips. A 'one' is replaced by a code word of at least 10 chips and a 'zero' is replaced by the inverse of the same 10-chip word.
- Hoppers use a sequence of jumps and short dwells at different fequencies. Usually several data bits are transmitted during each frequency stop, but ultra-fast hoppers will make several jumps during one data bit. Fast switching LO hoppers are more difficult technology than chippers, but they use less power.
- The spectrum is divided into 1 MHz wide channels for hoppers. Dwell time per channel is limited to 400 ms out of 20 seconds in the 900 MHz band and 400 ms out of 30 seconds in the 2.4 GHz band. You must hop through at least 50 channels at 900 MHz and 75 channels at 2.4 GHz.
- Spread spectrum modulation has good immunity to interference and causes less interference to others due to low spectral power density from multiple devices wsith the same code as in a WLAN, and from other sources (e.g. multipath reflections). But the interference is very much less serious than for other modulation.
- But you pay a price in needed bandwidth and additional signalling rates beyond the normal error checking used in digital modulation to synchronize data, send acknowledgements, etc. For example, and access point from Lucent Technologies direct sequence WLAN runs at 2 Mb/s, but 0.6 Mb/s is used for overhead. Proxim's hopper runs at 1.6 Mb/s and uses 1 Mb/s for overhead, so only 0.6 Mb/s remains for your data. You can add more access points at more money cost and increased possibility of interference.
- In any event, a large building needs many access points which cover a 30 to 300 meter radius depending upon wall types and obstructions. A lower frequency gives more range so 900 MHz is best, but the 900 MHz band is crowded so its not so good.
JMS, SA Seminar